Why did the Zimmerman note cause outrage among Americans

The Zimmerman note or the Zimmerman telegram was a telegram sent by the German foreign secretary to the German ambassador to Mexico. It proposed that, if the US were to ever enter into World War One, Germany should ally itself with Mexico. If Mexico were to give substantial financial aid to Germany, Mexico could recover the territories of Texas, New Mexico, and Arizona. This outraged the American public and set them even more firmly against Germany. So much so that one month after it was published, the US declared war on Germany.